sounds like you have a small leak. Probably at the fuel injector or hose near the engine and when you have the A/C on, it sucks the fumes into the cabin. Have it looked at ASAP, due to your safety of breathing in the fumes and potential risk of fire.
Do you have an exhaust leak? If you have an exhaust leak near the engine, you will have a strong smell upon startup or when running the vent/AC.
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